Calum MacLean, Ineos Grangemouth (UK) chairman, said: "Unite's response is unbelievable given how much effort has been put into securing this deal with ourselves, the Scottish and the UK governments all working hard to find a way forward.

"We will now concentrate on discussing the Survival Plan with our staff during the 60-day consultation".

Unite said it had already promised no strike action before Christmas but the union repeated its calls for Ineos to abandon proposed changes to staff contracts.

"This was on the basis that the company would not impose cuts on the workforce during the same period. Unite also proposed that these negotiations over the future of the site during this period would be held under the auspices of Acas."

He added: "If the company lifts the cuts agenda that it is imposing on its workers, then our offer still stands."

Proposals letter

Scotland's First Minister Alex Salmond earlier warned of a risk to the future of Grangemouth if the current stand-off continues.

Speaking on BBC Radio Scotland, he said the longer site remained "idle and cold" the more difficult things would become.

Mr Salmond met with Ineos and Unite representatives on Thursday.

Scottish Labour leader Johann Lamont has also become involved with the dispute, saying the Ineos management tactics were "not the way to engage in workplace disputes".

She called on bosses at the refinery to withdraw the ultimatums they had delivered to the workforce earlier.

Ineos said shutting down the Grangemouth complex had cost the company £20m so far.

The firm had put proposals over pay and pensions to workers at the complex, which has 1,400 employees and many more contractors.

A letter outlining the proposed changes - which includes freezing the basic salary and offering no bonuses until at least the end of 2016 - was being couriered to employees on Friday.

Unite claimed the company had told staff they could lose their jobs and be reemployed on poorer terms unless they agreed to give up their final salary pension, and accept pay freezes and pay cuts, by 18:00 on Monday.

The dispute that led to a vote for strike action had centred on the company's treatment of union official Stephen Deans.